Sheets of crystalline polyolefins such as polypropylene and polyethylene have excellent mechanical and thermal properties and sanitariness. Thus, those sheets have been widely used in fabrication of food trays, food containers, industrial mechanical parts, car parts, and so forth as substitutes for conventional sheets of polystyrene, an acrylonitrile/butadiene/styrene copolymer, polyvinyl chloride, and the like.
With extension of the range of use of such crystalline polyolefin sheets, various improvements have been made in the composition thereof for the purpose of greatly improving deep drawing properties at the time of vacuum or pressure forming. Typical improvements are described in, for example, Japanese Patent Application (OPI) Nos. 108433/80 and 142810/83 (the term "OPI" as used herein means a "published unexamined Japanese patent application").
A composition comprising a specific type of polyethylene and a specific type of polypropylene is used in Japanese Patent Application (OPI) No. 108433/1980, and a composition comprising specific types of polypropylene and polyethylene and a specific amount of a styrene polymer is used in Japanese Patent Application (OPI) No. 142810/1983. The use of these compositions broadens the molding temperature range of sheets and also permits a higher extent of deep drawing and production of product having reduced irregularities in thickness.
Products produced by vacuum or pressure forming the compositions, however, have several disadvantages. One of the disadvantages is that the gloss of the products is poor. In producing sheets, the compositions are contact cooled on a mirror surface roll to maintain, the roll surface state and as a result, sheets having excellent gloss can be obtained. These sheets, however, are re-melted at a preheating step for vacuum or pressure forming and are allowed to crystallize in a free surface state at a cooling step after the deep drawing, resulting in deterioration of the gloss thereof. Thus, those sheets are unsuitable for use in applications where the gloss is required.
Another disadvantage is that when inorganic fillers are compounded to the compositions so as to increase their heat resistance, stiffness and dimensional stability, the impact resistance of the resulting compositions is seriously reduced and sheets produced therefrom are unsuitable for practical use.